This invention relates to diagnostic x-ray apparatus and, in particular, to a system for compensating the kilovoltage applied between the anode and cathode of an x-ray tube during an exposure for power supply voltage variations. The new compensating system is especially useful in mobile x-ray units which use battery power supplies whose output voltage declines as the battery discharges but as will be evident hereafter, the system can also be used to compensate for line voltage variations in cases where the x-ray unit is supplied from the ac power lines in a building.
In most mobile x-ray units, batteries supply dc power to an inverter and the ac power output from the inverter is fed to an autotransformer to which the primary winding of a step-up x-ray tube anode supply transformer is connected. The ac output voltage from the secondary of the transformer is rectified and applied between the anode and cathode of the x-ray tube during an exposure. In prior art systems, before an x-ray exposure was made, the operator was required to observe a battery charge condition indicating meter and then manipulate a control which changed taps on the autotransformer windings, that is, the turns ratio of the autotransformer was changed, to yield an ac output voltage that was compensated for the source voltage, such as the battery voltage, being below fully charged level. This procedure allows possible human error to enter into battery voltage compensation.
Some rather sophisticated general purpose source voltage variation compensators have been developed which use digital logic techniques and which could be applicable to x-ray power supply voltage compensation. For example, U.S. Pat. No. 3,818,321 discloses a circuit wherein the ac voltage on the secondary of a power supply autotransformer is sampled and converted to a dc analog signal which is, in turn, converted to an equivalent digital value or code. The number of winding turns are controlled by controlling closure of one tap switch at a time in a digital fashion. Each tap switch corresponds to a unique count in a digital counter. The regulated voltage is compared with several reference voltages defining the regulation range. The comparison results are used in discrete steps to control the counts in the counter and thereby control closure of a particular switch so that the controlled voltage is between the reference voltage limits which occurs only when the regulated voltage is within the desired range. Complexity is one of the disadvantages of this system.
A variety of other voltage regulating systems for x-ray tube power supplies have also been developed which use digital logic techniques. Typically, these are based on use of a microprocessor. Signals representative of the voltage which the operator desired to have applied between the anode and cathode of the x-ray tube for making an exposure are fed to a microprocessor which executes a program that results in switching procedures being carried out for sending the proper applied voltage to the x-ray tube transformer. Systems of this kind, however, depend on independent means for regulating the supply voltage. They do not provide a simple means for compensating for source voltage variations automatically in coordination with the operator simply selecting the voltage which is desired to be applied to the x-ray tube for an exposure.